That’s the question behind CryptoKitties, a new service to buy, breed, and sell digital cats on the blockchain. These cats are more similar to real-world collectibles like Beanie Babies or baseball cards than they are to cryptocurrencies like Bitcoin or Ethereum. But unlike baseball cards, you can breed them.

In just the past few months, a whole community of third-party sites and services has formed around CryptoKitties. And fans have spent more than $23 million playing along.